Telephone transmission line



Patented Jan. 9, 1934 PATENT OFFICE TELEPHONE TRANSMISSION LINE Frederick M. Crapo, Muncie, Ind., assignor to Indiana Steel & Wire Company, Muncie, Ind., v a corporation of Indiana No Drawing. Application June 8, 1931 Serial N0. 543,024

My invention relates to long overhead telephone-transmission lines- I here use the term long to differentiate from mere river crossings or other relatively short and insignificant portions of transmission lines. By the term telephone transmission lines I mean transmission Claims.

lines which carry currents of the voice-frequency range, or of still higher frequencies, including not only those which are used for the transmission of speech but also those which are used for other forms of communication, such as high-speed machine telegraphy, carrier telephony, and carrier telegraphy.

It is the object of my invention to improve the ferrous conductor used in telephone transmission as considered from the standpoint of tensile strength, and yet to avoid impairing it from the standpoint of conductivity for currents of the voice-frequency range. In accomplishing this, I make it possible to lessen the cost of installing and maintaining a telephone line of ferrous conductor, by lengthening the spans and thus reducing the overall cost of construction and maintenance, while still maintaining both the structural strength of the line as a whole and its transmission eificiency for such currents of the voice-frequency range.

Heretofore the ferrous conductors in general use for telephone lines have been of three general commercial grades. These are known as E. B. B. (Extra Best Best), B. B. (Best Best), and Steel. given in the order of desirability from the standpoint 'of electrical conductivity for ,direct currents, so that they are in the order of increasing electrical D. C. resistivity; but in these grades the advantages of high D. C. conductivity or low resistivity have been obtained at the sacrifice of tensile strength, so that the foregoing order of the three grades also indicates the order of their increasing tensile strength. As a result, the wires of these three grades have minimum breaking loads which are substantially 2.5, 2.8, and 3.0 times the weight of the' respective wires per mile, or minimum tensile strengths of ap proximately 45,000 pounds, 51,000 pounds, and 95,000 pounds per square inch of cross-section. While these minimum breaking loads and tensile strengths are of course slightly exceeded in actual practice, none of these commercial grades of wires has a tensile strength much if any in excess of 63,000 or 64,000 pounds per square inch or a breaking load much if any in excess of 3.5 times the weight of the wire per mile, and only very rarely has such a wire a tensile strength in These three commercial grades are excess of 60,000 pounds per square inch or a breaking load in excess of about 3.3 times the weight of the wire per mile.

In all these three grades of wire, the carbon content has usually been less than 0.15%. All three grades have also had varying small amounts of manganese, silicon, sulphur, phosphorus, and sometimes copper, in quantities such as are met in mild steels of commercial grades; and so may wire involving my invention.

Ordinarily these ferrous telephone conductors of all three grades are annealed wires; and are almost universally galvanized wires, galvanized after being annealed.

At the present time, and for many years past, telephone engineers have considered that B. B. wire represents the best commercial compromise between desirably low electrical resistivity and desirably high tensile strength.

Only under some very special conditions, as in a long span across a river, has tensile strength been made a .primary consideration; and then only because of necessity, as in order to get sufllcient tensile strength for a long span which could not be avoided. When wire of higher tensile strength was thus used under those special conditions, it was used for as few spans and foras small a part of the whole circuit as possible; because it was considered that the increased tensile strength could not be obtained without impairing the telephonic transmission efliciency of the conductor.

I have discovered, however, that it is possible to increase the tensile strength materially without impairing the eificiency of the ferrous conductor for transmitting currents of the voicefrequency range. That is, I can add materially to the tensile strength of the wire, easily up to a value of more than double and sometimes even almost quadruple that of B. B. wire, and yet maintain a transmission efllciency of the conductor for currents of voice frequency which is substantially equal to that of B. B. wire. I can and in most cases do obtain the increase in tensile strength without serious sacrifice in ductility.

I accomplish this by choosing a material for the ferrous-conductor wire on the basis of effective alternating-current resistance for currents of voice frequency, instead of on the basis of simple (D. C.) resistivity; which makes it possible to obtain a much higher tensile strength. With such a material, the simple (D. C.) resistivity may be higher than is permissible for the present commercial grades above mentioned; but when that occurs I still keep the effective A. C. resistivity to currents of voice frequency sufficiently low so that it is not only satisfactory but is substantially of the same order as that of B. B. wire. Instead (of using this wire of higher tensile strength merely as river crossing wire is used, for as small a part of the circuit and as few spans as possible, I deliberately use it for the whole or at least a large part of the transmission line, so that it forms a multiplicity of spans of the overhead construction in which ferrous conductor is commonly used.

This increase in tensile strength without a corresponding increase in effective A. C. resistivity for currents of the voice-frequency range may be obtained in various ways. I prefer to obtain it by raising the carbon content of the ferrous material, to a value above 0.25%, but desirably below the eutectoid value so that the material will be hypo-eutectoid carbon steel. I may also raise the tensile strength in other ways, as for instance by cold-working, regardless of the effect thereof on simple (D. C.) resistivity, so long as Iraise the tensile strength above the desired point while maintaining the effective A. C. resistivity to currents of voice frequency below the desired point. These things may, and in most cases do, result in lowering the skin-effect ratio the ratio between the effective A. C. resistance and the simple or D. C. resistancebut that is advantageous for currents of the voice-frequency range.

By thus increasing the tensile strength, without correspondingly increasing the effective A. C. resistivity to currents of voice frequency, I am able to get a materially higher ratio between the tensile strength and such effective A. C. resistivity.

Since electrical resistance, whether A. C. or D. 0.. is measured in ohms, it is necessarily dependent upon the size of the wire. So is the ratio between the effective A. C. resistance and the simple or D. C. resistance. For the sake of setting this forth, both in the specification and in the claims, I have made my comparisons with galvanized wires of No. 12 B. W.,G. size. Also, since'the A. C. resistance of ferrous wire depends on the current-value, I have taken five milliamperes as a fair epresentative value of the voice-produced alternating currents that are carried by the transmission line. voice frequencies vary over a considerable range, and the A. C. resistance and the skin-effect ratio vary with the frequency of the transmitted A. C. current, I have taken 1000 cycles per second as a fair single-equivalent voice frequency. These various values are used in the claims indeflning .the invention.

Further, in ferrous conductors now in general use for telephone lines a considerable degree of so-called modulation is inherent. This results from the fact that the rate of attenuation is different for different frequencies. That'is, the effective resistance to alternating currents increases with'the frequency; so that there is a greater loss of energy with currents of higher frequencies than there is with currents of lower frequencies. As a result, there is a distortion in quality in the transmission of voices and other sounds, with an impairment in intelligibility and in faithfulness of reproduction. By using wire of higher tensile strength, and making possible the obtaining of a lower skin-effect ratio, I am able to decrease this modulation, and so to reduce this impairment of intelligibility and increase the faithfulness of sound reproduction.

The ferrous conductors which accord g 9 my Further, since invention form a large part of a long overhead telephone-transmission circuit, comprising a multiplicity of spans of overhead conductor, are those which have some one or more of the following characteristics as set forth in the various claims: 7

a. The wire is of ferrous material, especially of carbon steel, and most particularly a hypoeutectoid carbon steel.

b. It contains over 0.25% carbon.

0. It has, when in galvanized wire of No. 12 B. W. G. size, and especially when in substantially annealed state, an effective A. C. resistance of not more than 12 ohms per thousand feet to alternating currents of 1000 cycles per second and 5 milli-amperes.

cl. It has, when in galvanized wire of No. 12 B. W. G. size, a skin-effect ratio, or ratio between its effective A. C. resistance to alternating currents of 1000 cycles per second and 5 milli-amperes and its simple or D. C. resistance, of not more than 1.35.

e. It has a tensile strength of not less than 70,000 pounds per square inch; and may range from that value up to a value in the neighborhood of 200,000 pounds per square inch, or even more.

I. It has, especially when in substantially annealed state, a tensile strength in pounds per square inch that is at least 6800 times as great as its effective A. C. resistance in ohms per 1000 feet in galvanized wire of No. 12 B. W. G. size with currents of 1000 cycles per second and 5 milliamperes; and such tensile strength may be much larger than that, even up to and exceeding 18,000 times that effective A. C. resistance.

a. It has, when in galvanized wire, a minimum breaking load of 3.8 times its weight per mile.

In the following table I show some comparisons in these points between a sample of the present commercial B. B." telephone wire on the one hand, and some examples of telephone wires embodying my invention on the other hand, as obtained from actual measurements with galvanized wires of No. 12 B. W. G. size, the alternating currents used being of 1000 cycles per second and 5 milli-amperes.

- Effcc- Ratio of Rcsisttive skim tensile A present commercial in 5 g, g 'gl efiect telephone wire; less pounds in d anc'w resistfive than 0.15% carbon per 6' h ance- & C

square per m 0 ms ratio inch feet per 1000 resistfeet ancc Examples of telephone wires of this application Wire of0.425% carbon 95, 000 8. 53 10. 03 1.175 9471 Wire ofO.6l2% carbom. 199, 000 1 10.00 10.65 1. 065 l8085 3. A long overhead telephone-transmission line, including as at least a large part of it-a conductor made of carbon steel which when in the form of a galvanized wire of No. 12 B. W. G. size has a skin-effect resistance ratio of not more than 1.35 for alternating currents of 1000 cycles persecond and 5 milli-amperes and which contains not less than 0.25% of carbon.

4. A long telephone-transmission line, including as at least a large part of it a carbon-steel wire containing not less than 0.25% of carbon.

5. An overhead telephone-transmission line, comprising a series of many spans of ferrous wire made of steel which contains not less than 0.25% of carbon and which when in the form of a galvanized wire of No. 12 B. W. G. size has an effective A. C. resistance of not more than 12 ohms per 1000 feet to alternating currents of 1000 cycles per second and 5 milli-amperes.

6. A long overhead telephone-transmission line, including as at least a large part of it a conductor made of ferrous material which when in the form of a galvanized wire of N0. 12 B. W. G. size has an effective A. C. resistance of not more than 12 ohms per 1000 feet to currents of 1000 cycles per second and 5 milli-amperes and which when in substantially annealed state has a tensile strength in pounds per square inch that is at least 6800 times as great as its said effective A. C.- resistance.

7. An overload telephone-transmission line, comprising a series of many spans of wire of ferrous material which when in the form of a galvanized wire of No. 12 B. W. G. size hasan effective A. C. resistance of not more than 12 ohms per 1000 feet to currents of 1000 cycles per second and 5 milli-amperes, and which has a tensile strength in pounds per square inch that is at least 6800 times as great as its said effective A. C. resistance.

8. An overhead telephone-transmission line, comprising a series of many spans of wire of ferrous material which when in the form of a galvanized wire of No. 12 B. W. G. size has an effective A. C. resistance of not morethan 12 ohms per 1000 feet to currents of 1000 cycles per second and 5 milli-amperes and which when in a substantially annealed state has a tensile strength in excess of 70,000 pounds per square inch.

9. A long overhead telephone-transmission line, including as at least a large part of it a conductor made of ferrous material which when in the form of a galvanized wire of No. 12 B. W. G. size has an effective A. C. resistance of not more than 12 ohms per 1000 feet to currents of 1000 cycles per second and 5 milli-amperes and which has a tensile strength in excess of 70,000 pounds per square inch.

10. A long overhead telephone-transmission line, including as at least a large part ofit ,a conductor made of ferrous material which when in substantially annealed state has a tensile strength in excess of 70,000 pounds per square inch, which contains not less than 0.25% of carbon, and which when in the form of a galvanized wire of No. 12 B. W. G. size has an effective A. C. resistance of not more than 12 ohms per 1000 feet to alternating currents of 1000 cycles per second and 5 milli-amperes.

11. An overhead telephone-transmission line,

comprising a series of many spans of wire of fer-' rous material which has a tensile strength in ex cess of 70,000 pounds per square inch, which contains not less than 0.25% of carbon, and which when in the form of a galvanized wire of No. 12

'B. W. G. size has an effective A; C. resistance of not more than 12 ohms per 1000 feet to alternating currents of 1000 cycles per second and 5 mini-- in excess of 10,000 pounds per square inch, and which when in the form of a galvanized wire of No. 12 B. W. G. size has a skin-effect resistance ratio of not more than 1.35 for alternating currents of 1000 cycles per second and 5 milll-amperes.

14. A long overhead telephone-transmission line, including as at least a large part of it a conductor made of hypo-eutectoid carbon steel which when in the form of a galvanized wire of No. 12 B. W. G. size has a skin-effect resistance ratio of not more than 1.35 for alternating cur rents of 1000 cycles per second and 5 milli-amperes and which when in a substantially annealed state has a tensile strength in pounds per square inch that is at least 6800 times as great as the effective A. C. resistance which said galvanized wire made of it has in ohms per 1000 feet to said alternating currents.

15. An overhead telephone-transmission line, comprising a series of many spans of wire of .stantially annealed state has a tensile strength hypo-eutectoid carbon steel which has a tensile strength in pounds per square inch that is at least 6800 times as great as the efiective A. C. resistance which a galvanized wire made of it of No. '12 B. W. G. size has in ohms per 1000 feet less than 0.25% of carbon and which when in substantially annealed state has .a tensile strength in pounds per square inch that is at least 6800 times as'great as is the effective A. C. resistance which a galvanized wire made of it of No. 12 B. W. G. size has in ohms per 1000 feet to currents of 1000 cycles per second and 5 milliamperes.

17. A long overhead telephone-transmission line, including as at least a large part of it a ferrous wire made of steel which contains not less than 0.25% of carbon and which when in a substantially annealed state has a tensile strength in. excess of 70,000 pounds per square inch.

FREDERICK M. CRAPO.

CERTIFICATE of CORRECTION Patent No. 1,942,411. January 9, 1934. FREDERICK M. CRAPO It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page .1, line 47, for the blurred number before pounds read 55 ,000; page 2, the left-hand vertical column appearing in lines 123to 137 inclusive, should appear as shown below instead of as shown in the patent A present commercial telephone wire; less than 0.15% carbon HB.BIH

Examples of telephone wlres of this .application Wire of 0.425% carbon Wire of 0.612% carbon I page 3, line 31, claim 7,for overload read overhead; and that the stud Letters Patent should be read with these corrections therem that the .same may conform to the record of the case in the Patent Office.

Signed and sealed this 6th day of March. A. D. 1934.

SE L F. M. HOPKINS,

' Acting Commissioner of Patents. 

